It is sometimes necessary to provide cavities in such a wafer having a substantially flat bottom with different depths. Although it is obviously possible to obtain such cavities sequentially, it may be readily seen that such a solution is not advantageous since it requires as many masking operations and as many etching operations as there are cavities to be obtained .
U.S. Pat. No. JP-A-58/98927 describes an etching method enabling the simultaneous obtaining of cavities having different depths. This method comprises an anisotropic etching step followed by an isotropic etching step. During the first step, the cavities are formed with inclined sides, the etching stopping as soon as the sides come together. It is thus understood that the maximum depth of such a cavity is a function of the surface dimension of the latter. The second step (isotropic etching) has as effect to increase the depth of all the cavities to the same amount.
This method shows a certain number of disadvantages. First of all, it is not possible to form two cavities of the same surface area and of different depths, nor a deep cavity of small surface at the same time as a shallow cavity of large surface, since the etching speed is the same for all cavities and the maximum depth during the first step is directly proportional to the surface of the cavity.
Furthermore, this method does not enable obtaining a flat surface in all cavities, certain among them having to the contrary a V-shape bottom.
Next, this method does not enable one to control exactly the depth of all the cavities Effectively, as has been shown in FIG. 1 of the above mentioned document, if one wishes to form two cavities having different surface areas exhibiting between themselves a difference in depth less than the difference between their respective maximum depths, it is necessary to stop the etching of the cavity having the greatest surface before its maximum depth is attained. Thus, the moment when it is necessary to stop the etching may not be determined in a particularly precise manner since the etching speed can only be roughly estimated.